1. Field of the Invention
The present invention relates to a tolan compound which is useful as a liquid crystal material and to a liquid crystal composition containing this tolan compound. The present invention also relates to a liquid crystal display device using the liquid crystal composition containing this tolan compound.
2. Background Art
A liquid crystal display device has been widely utilized for a watch, an electronic calculator, a word processor, a television set, and the like. Of these liquid crystal display devices, particularly frequently used is a liquid crystal display device which utilizes optical anisotropy and dielectric anisotropy of a liquid crystal material.
A wide liquid crystal temperature range, a low viscosity to ensure a rapid electro-optical response rate, a wide visual range, a suitable double refractive index (.DELTA.n) to provide a wide visual scope and a high contrast, a large dielectric anisotropy (.DELTA..epsilon.) to ensure a low driving voltage, and the characteristics to be chemically and optically stable are given as the characteristics required for a liquid crystal material to be used for liquid crystal display devices.
Liquid crystal compositions presently used are usually prepared by mixing a compound having a liquid crystal phase near room temperatures and a compound having a liquid crystal phase at temperatures higher than room temperatures. In order for a liquid crystal display device to be used outdoor the liquid crystal must be stable in the temperature range of -40.degree. C. to 90.degree. C. In addition, because of the temperature dependency of dielectric anisotropy and refractive index anisotropy, that is, because of occurrence of sudden change near the N-I point (nematic anisotropy phase transition temperature), it is necessary to use a liquid crystal material with a high N-I point.
Furthermore, to ensure a wide visual angle and a high contrast, the retardation of liquid crystal phase, .DELTA.n.multidot.d, wherein .DELTA.n is the double refractive index of the liquid crystal material and d is the thickness of liquid crystal layer, must be optimized. However, because the thickness of liquid crystal layer (d) is limited to a certain range and a high response rate is required for liquid crystal display devices actually used, the thickness of liquid crystal layer (d) tends to be thin. Because of this reason, a liquid crystal material with a large double refractive index (.DELTA.n) is required.
A liquid crystal material is also desired to have a low viscosity to achieve high speed operation. A rise time, indicated by .tau..sub.on, when voltage is applied, and a fall time, indicated by .tau..sub.off, when voltage is cut off, are given by the following equations: EQU .tau..sub.on =.eta..sub.ii d.sup.2 (.epsilon..sub.0.DELTA..epsilon.V.sup.2 -K.pi..sup.2).sup.-1 EQU .tau..sub.off =.eta..sub.ii d.sup.2 /.pi..sup.2 K
wherein .eta..sub.ii is a viscosity parameter, d is a thickness of a liquid crystal layer, .epsilon..sub.0 is a dielectric constant in vacuum, .DELTA..epsilon. is an anisotropy of dielectric constant, V is an applied voltage, and K is a parameter in relation to a modulus of elasticity, which is represented by the equation: EQU K=k.sub.11 +(k.sub.33 -2k.sub.22)/4,
in which k.sub.11, k.sub.22, and k.sub.33 are spray, twist, and bend modulus of elasticity respectively. Accordingly, in order to obtain a high response rate, the liquid crystal material must have a low viscosity. Use of a liquid crystal compound having a low viscosity is therefore indispensable.
A driving voltage is dependent on a threshold voltage Vth, whereas the threshold voltage Vth is inversely proportional to the square root of dielectric anisotropy (.DELTA..epsilon.) Accordingly, the threshold voltage Vth can be controlled low when a liquid crystal material with a positive value of dielectric anisotropy (.DELTA..epsilon.) is used.
Although various liquid crystal compounds have been developed and put on use up to the present time, there is no single liquid crystal compound satisfying all the characteristics mentioned above. Therefore, in practice, several kinds of liquid crystal compounds having different characteristics or non-liquid crystal compounds are mixed and used. Such mixtures, however, are not necessarily satisfactory.
4-Alkyl-4'-alkoxytolan (German Patent No. 2226376) and 4-alkyl-4'-fuluorotolan (Japanese Patent Application Laid-Open No. 260031/1986) are examples of known tolan compounds which are used mixed for the above-mentioned purpose. The mixtures of these tolan compounds, however, are not necessarily satisfactory.
As described above, in spite of various studies in the past on liquid crystal compositions, no material which sufficiently satisfies all above characteristics has been found. Each compound used for liquid crystal materials has merits and demerits in its characteristics. In addition, the degree of the above required characteristics is different according to the type of liquid crystal display devices. The development of a novel liquid crystal compound and a liquid crystal additive which can provide characteristics according to the intended object of a liquid crystal display device has been desired.
Accordingly, an object of the present invention is to provide a novel tolan compound which, if added to a liquid crystal composition, can raise the N-I point, reduce the viscosity, and increase the double refractive index (.DELTA.n) of the liquid crystal composition. Another object of the present invention is to provide such a liquid crystal composition comprising this novel tolan compound. Still another object of the present invention is to provide a liquid crystal display device using the liquid crystal composition.